Well, after calling into the police station to chase things up a couple of weeks ago, the PC in charge got back to me. Apparently the person who owns the car is a good bit older than the people in the car when I was slapped. They visited the owner, but since no-one admitted driving the car they couldn't take it any further; without hard evidence, this is what I had expected. I imagine it was the son of whoever owned the car, and I just hope that a uniformed copper knocking on the door gave them as much of a nasty surprise as the original incident did to me.
I'm still a bit wary cycling along that road particularly as it turns into a 30MPH descent a little bit further on, but I haven't seen the offending car since.